The present invention is generally related to devices and methods for controlling and converting laser energy wavelengths, and in a particular embodiment, provides a passive temperature compensation system for a nonlinear optic.
Lasers have been used for several years to sculpt materials into very precise shapes, excimer lasers are now widely used to ablate tissue in a variety of surgical procedures, particularly for corneal ablation during refractive surgery. The exposure of the tissue is typically controlled to produce a desired change in corneal shape. The change in corneal shape may be intended to correct a refractive error of the eye so as to eliminate the need for corrective eye glasses, or may be intended to remove a pathology from the eye.
Known laser eye procedures generally employ ultraviolet or infrared lasers to remove a microscopic layer of stromal tissue from the cornea to alter its refractive characteristics. The laser often has a frequency selected to result in photodecomposition of the corneal tissue, preferably without causing significant thermal damage to adjacent and underlying tissues of the eye. These selected frequencies can break the radiated molecules into smaller volatile fragments photochemically by directly breaking the intermolecular bonds. These known refractive lasers often deliver laser energy as a series of discrete energy pulses, with each pulse having sufficient energy to ablate a thin volume from adjacent the corneal surface. The refractive surgical system generally control the distribution of the ablative laser energy across the cornea using, for example, ablatable masks, movable apertures, scanning systems that move the laser across the corneal surface, combinations of these techniques, and the like.
An exemplary system and method for sculpting a cornea by controlling a plurality of laser beams is described in co-pending U.S. patent application Ser. No. 09/274,499 as filed on Apr. 23, 1999, the full disclosure of which is incorporated herein by reference.
While known laser eye surgery systems have been found to be highly effective, as with all successes, still further improvements would be desirable. In particular, known laser eye surgery systems often rely on excimer lasers to produce laser energy in the deep ultraviolet wavelengths. To produce this laser energy, these excimer lasers often make use of gases such as argon-fluoride to produce a beam having a wavelength of about 193 nm. Although such excimer lasers are highly effective, there are significant maintenance costs associated with consumption of gases in the laser. Servicing costs and the lifetime of the laser chamber are less than ideal, while cleaning and replacement of the optical components is more often than would be desired.
Solid-state lasers have a number of desirable characteristics. For example, these lasers may allow higher repetition rates than excimer lasers. Solid-state lasers may also cost less and have a longer useful life than an excimer laser. Unfortunately, solid-state lasers generally do not provide highly coherent radiations in the deep ultraviolet wavelengths, which are desirable for ophthalmic surgery and for other applications including semiconductor processing, diagnostic applications, and the like.
It has previously been proposed to make use of solid-state lasers for refractive surgery and other applications by converting the laser output wavelength to a more desirable frequency using Non-Linear Optics (sometimes referred to as NLO""s). Non-Linear Optics generally produce energy which is significantly different than the radiation incident thereon. Non-Linear Optics include beta barium borate, lithium triborate, cesium lithium borate, periodic pooled lithium niobate (LiNbO3), and other materials such as RTA, RTP, GaAs, KTA, KTP, LiTaO3, lithium tantalate, and the like. These and other nonlinear crystals can be used to convert laser energy having an initial wavelength to an alternative laser energy having a wavelength which is a harmonic of the initial wavelength, for example, by doubling a frequency of the laser energy. These and other nonlinear crystal materials may also be used to combine two or more differing laser input energies to produce an output energy of a desired wavelength, for example, by mixing the input laser energy so as to sum frequencies for the output laser energy. An exemplary method and system for producing coherent deep ultraviolet output from a solid state laser is described in U.S. Pat. No. 5,742,626 issued to Mead et al., the full disclosure of which is incorporated herein by reference.
While the frequency multiplied and sum-mixed outputs of the proposed ultraviolet solid-state laser systems provide significant potential advantages for use in laser eye surgery, semiconductor fabrication, and other uses, these proposed solid-state systems have their own disadvantages. In general, the energy conversion provided by Non-Linear Optics can vary significantly with temperature. More specifically, the angle of incidence for efficient phase matching and optical frequency conversion in a NLO may be a function of the temperature of the NLO.
In known devices in which NLO""s are used to change the frequency of the laser beam, for example, second or third harmonic generation, the sum-difference mixing of two beams, or the like, the temperature of the crystal is often actively controlled so as to maintain the desired conversion characteristics. In other known systems using NLO""s, the angle of incidence is actively changed by providing a control signal to a motor coupled to the NLO so as to rotate the NLO in response to sensed temperature changes. Both of these known active NLO temperature compensation systems rely on monitoring of sensor data, feeding back the sensor data to a control system, and varying the control mechanism (either temperature or angle) of the NLO so as to maintain the desired energy output. These complex feedback systems increase the complexity of the cost of the previously proposed solid-state, deep ultraviolet systems, significantly mitigating their potential advantages over more common alternatives, such a excimer lasers.
In light of the above, it would be desirable to provide improved laser systems, methods and devices. It would be particularly beneficial to provide improved techniques and systems for maintaining and/or controlling the output of NLO""s, especially if these improved techniques avoided relying on active (and often expensive) feedback and control systems. The devices, systems, and methods of the present invention at least partially mitigate the disadvantages of known solid-state laser systems, and thereby realize some or all of these improvements.
The present invention generally provides improved devices, systems, and methods for converting radiant energy. The systems of the present invention generally make use of a Non-Liner Optic (NLO) to effect a conversion of an input laser energy to an output energy. The output energy will often have a wavelength which is different than the input energy. The conversion provided by the NLO will often vary in response to both an angle of the energy relative to the NLO, and in response to a temperature of the NLO. The present invention generally provides passive control over the angle of the NLO based on thermal expansion of a member which is thermally coupled to the NLO. Advantageously, the thermal-expansion induced change in angle of the NLO can compensate for the temperature-induced change in the conversion so as to maintain a desired output frequency, conversion efficiency, phase matching, and/or the like.
In a first aspect, the invention provides a laser system comprising a laser generating a laser beam with a first frequency. A NLO is disposed in an optical path of the beam. The NLO effects a conversion of the first frequency to a second frequency. The conversion varies with an angle of the NLO relative to the optical path. A first member has a first thermal coefficient of expansion and is thermally coupled to the NLO so that a change in a dimension of the first member with a change in temperature of the NLO effects a change in the angle of the NLO.
Preferably, the change in dimension of the member effects a predetermined change in the angle of the NLO so as to effect a desired adjustment in the conversion. Typically, the angle-induced adjustment in conversion will compensate for temperature-induced changes in the conversion. The NLO will often pivot within the optical path in response to the passive expansion by the member so that the second frequency remains within a desired (often predetermined) range when a temperature of the NLO varies throughout a predetermined temperature range during operation of the laser system.
In the exemplary embodiment, first and second members having differing coefficients of thermal-expansion are attached together. Differing thermal expansion of the attached members alters a bend angle of the members, and the bend angle pivots the NLO within the optical path. Optionally, an angle adjustment mechanism may allow varying of the NLO angle independent of temperature for calibration and adjustment of the system. Suitable adjustment mechanisms may make use of structures similar to micrometer linear scale systems.
In another aspect, the invention provides a laser eye surgery system comprising a laser generating a laser beam with a first frequency. A NLO is disposed in an optical path of the beam so as to define an angle relative to the beam. The NLO effects a conversion of the first frequency to a second frequency. The conversion exhibits an angle-induced change with a change in the angle, and a temperature-induced change with a change in a temperature of the NLO. A compensator includes a first member having a thermal coefficient of expansion. The first member is thermally coupled to the NLO so that the change in temperature of the NLO effects a change in a dimension of the first member. The first member is mechanically coupled to the NLO. The change in dimension of the first member effects the change in angle of the NLO so that the angle-induced change in the conversion compensates for the temperature-induced change in the conversion. A beam directing system is disposed in the optical path from the NLO. The beam directing system selectively directs the beam toward portions of a cornea so as to effect a desired change in a refractive characteristic of the cornea.
In a method aspect, the invention provides a method comprising generating a laser beam at a first frequency with a laser. The laser beam is converted to a second frequency with a NLO. The converting step varies with a temperature of the NLO, and with an angle defined by the NLO and the laser beam. Temperature-induced variations in the NLO are passively compensated for by transferring heat to a member from the NLO. Thermal expansion of the member resulting from the transfer of heat adjusts the angle of the NLO.